Electrophotographic color printing



United States Patent Int. Cl. G03g 13/02; B41m 1/14 US. Cl. 961.2 8 Claims ABSTRACT 0F THE DISCLOSURE This invention relates to a process for the production of multi-colored prints by electrophotographic methods in which a set of at least two electrophotographic materials, each of which comprises a photoconductive layer containing a sensitizer which causes the layer to be lightsensitive only in a selected portion of the spectrum of visible light, are electrostatically charged, separately exposed to the same light image of a colored original to be reproduced, developed, fixed, and converted into printing plates, and the latter are printed over each other on a print-receiving material using printing inks, the colors of which are determined by the selected portions of the spectrum.

Processes for the preparation of color reproductions are known. In these processes, light rays going out from a colored original are restricted by an optical filter to the color range desired for the respective color separation. This color-restricted quantity of light strikes a lightsensitive reproduction material which is then converted, in conventional manner, into a printing plate. A color separation corresponding to this color results. Further color separations are prepared in an analogous manner with the same reproduction material and all the printing plates obtained therefrom are printed over one another with the specific color. A colored, printed image is formed which corresponds to the original. The light-sensitive material also may be an electrophotographic material.

A notable disadvantage of the foregoing process is that relatively complicated-optical devices, such as color filters, must be used. Apart from the additional work and equipment involved, the ever-present source of error of the optical devices have a notably unfavorable effect through deviations with regard to image sharpness.

It is also known to achieve from a panchromatically sensitized electrophotographic material, by putting an optical filter in front of it and exposing it under a colored original, a charge which corresponds to the respective color separation. After development of this latent image with a toner powder corresponding to this color, the process is repeated with further color filters and the same material, which already carries a color image, by means of which a color reproduction can be obtained on this electrophotographic material. A great disadvantage here is that the chargeability of an electrophotographic image which has already been treated with toner causes numerous difliculties from which a distorted or inadequate reproduction is generally to be expected. In addition, the preparation of a sizable number of reproductions according to this process is extremely time-consuming, since in the case of each reproduction many individual electroa photographic processes must be performed.

In the electrophotographic field, it is also already known to sensitize the photoconductive coatings, i.e., to render them light-sensitive, in such a way that they are sensitive to radiation only in a certain color range.

The present invention provides a process for the preparation, from colored originals using uncomplicated electrophotographic materials or sets of material without the use of optical color filters, of a set of printing plates which permits the inexpensive preparation of color prints according to the known color printing processes.

The invention, more specifically, provides a process for the preparation of colored prints by means of electrophotographic methods in which at least two electrophotographic materials suitable for printing purposes, whose photoconductive coatings are in each case through addition of sensitizers light-sensitive in only one part of the visible light spectrum, are exposed, after charging, with the colored original to be reproduced and then, in conventional manner, developed, fixed, and converted into printing plates. These plates are then printed over one another with the colors corresponding to the selected ranges of the spectrum.

The invention also provides a material for use in the process of the invention, comprising a set of at least two electrophotographic materials suitable for printing purposes, whose photoconductive coatings are in each case light-sensitive, due to the presence of at least one sensitizer, in only one part of the visible light spectrum.

The invention renders it possible, in an advantageous manner, to dispense with the use of interposed optical color filters. This simplifies the printing process substantially. By the use of the present set of printing material there is achieved, by simple, successive exposure of the individual electrophotographic materials and corresponding inking up of the printing plate prepared therefrom, faithful reproduction in the printing over one another which follows. Very long runs can be achieved with the printing plates prepared in this simple manner.

By electrophotographic materials which are suitable for printing purposes is meant materials which comprise, in conventional manner, a support suitable for printing purposes, such as an aluminum foil or a corresponding printing plate on a paper base, and a photoconductive coating containing one or more organic or inorganic photoconductors applied to the support, which photoconductive coating also contains additives controlling the lightsensitivity, i.e., so-called sensitizers.

By photoconductors is meant inorganic and organic substances usable for electrophotographic purposes, in particular oxazoles, oxadiazoles, thiodiazoles, quinazolines, triazines or polymeric compounds, such as polyvinylcarbazoles, or polyacrylates, or inorganic substances, especially zinc oxide. As sensitizing dyes, one or several dyes may be used in each case. The choice of the sensitizing dyes, which are added to the photoconductive coatings, is explained below.

One very satisfactory method consists in matching the chosen dyestutf with the printing inks used later in the printing process for the individual color separations. The reflectance range of these printing inks should be in close relationship with the range of the spectral sensitivity in the electrophotographic material caused by the sensitizing. The reflectance curve should lie with its maximum or with the largest part outside the range of spectral sensitivity of the sensitizer and the sensitivity curve of the sensitizer should if possible encompass the entire range of the visible spectrum except the reflectance range, i.e., the valuesof the spectral light-sensitivity should for each wavelength be inversely proportional to the reflectance values of the corresponding printing ink. This is further explained with the aid of an example of the inks most used for printing purposes:

If the blue color component is to be printed with the blue dye Cyan Blue BWD, C.I. 74160, the reflectance curve of the dye lies approximately in the range between 400 to 550 mg. The sensitivity curve of the sensitizing dye should then lie approximately between 550 mp. and the limits of long-wave visible light. Dyes which encompass this range when e.g. oxadiazoles and Lytrons are used, are e.g.

Brilliant Green-C.I. 42040 (approximately 560/700 m Crystal VioletC.I. 42555 (approximately 550/650 m r),

Malachite GreenC.I. 42000 (approximately 550/700 e). or

Victoria Pure Blue B--C.I. 44045 (approximately 510/ If the reflectance range for the yellow color component of the printing ink lies approximately between 490 to 750 m one or several suitable sensitizing dyes must be chosen which render the coating light-sensitive in the range from approximately 500 m to the limit of short-wave visible light (about 400 m Sensitizers which cause light sensitivity in this range are e.g. Thioflavine TG, C.I. 49005, Acriflavine, C.I. 46000, Acridine Yellow G, CI. 46025 and trypaflavine. These dyes cause light-sensitivity in the stated range only.

According to the same principles as above, when the red color component magenta is used, the pertinent sensitizing dye is chosen. As sensitizing dye, in this case a mixture of Rhodamine B extra, CI. 45170, Acridine Orange NO, CI. 46005 and Thioflavine TG, CI. 49005 may be used. The concentrations of these dyes are chosen in accordance with their sensitizing action, according to the reflectance curve of the magenta print ing ink in the manner described above (inverse proportion w In the same way, according to the printing inks used or intended to be used, whose reflectance ranges can easily be measured by conventional methods, the dyes which correspond therewith in the manner stated can be chosen. In addition, dyes which sensitize in a certain range are known from US. Patents Nos. 3,052,540 and 3,060,019.

A further method which permits good results for the choice of the sensitizers consists in matching the sensitivity range of the sensitizer as exactly as possible with the reflectance range of the printing inks to be used later. The methods of selection can be determined from what is stated above. This latter method naturally renders necessary the use of a reversal developer in the subsequent process steps.

The concentrations of the sensitizers can be varied without affecting the image quality. By the use of greater additions of dye, shorter exposure times result and, by the use of smaller proportions of dye, correspondingly longer exposure times are required. An upper limit to the amount of dye is determined by dark discharge of the charged coating. A lower limit is determined by the sensitivity of the unsensitized coating. The sensitivity caused by selective sensitizing must naturally, and which is also practically always the case, lie above the sensitiv ity of the coating itself in order to achieve the effect of the invention. Photoconductive materials most advantageously can be used whose own sensitivity does not lie, or only a small part of which lies, in the visible spectrum. As is known, the dyes do not behave equally with regard to their sensitizing properties when dilferent photoconductors are used.

According to the invention, the individual electrophotographic base materials are coated in each case with a photoconductor/sensitizer mixture which is different with regard to the sensitizer. These precoated materials, joined together, form the set according to the invention for printing purposes. The invention is further explained as follows, with reference to the process.

The colored original to be reproduced is exposed to irradiation with light and thus a color photograph is produced. To begin with, the first, previously charged, electrophotographic material, sensitized e.g. in the range of approximately 550 to 750 Ill .4, is irradiated by means of contact or episcopic or diascopic methods. The electrostatic charge decays in the light-sensitive range, i.e. between 550 and 800 mg. There remains a latent electrostatic image which corresponds to the blue portions of the original. In the same way, the subsequent electrophotographic materials sensitized for other spectral ranges are irradiated with the same photograph.

The developing of the latent images is now effected in conventional manner by means of toners which settle on the areas carrying the charge. The color of the toner is immaterial here. The powder images thus obtained are fixed thermally or by solvent vapors and inked up with the colors which correspond to the originally non-lightsensitive ranges of the spectrum. Marginally sharp colored prints with accurate color reproduction can be obtained by conventional printing over one another of the individual colors on a print-receiving material.

It is, however, also possible to expose the individual materials of the set of printing plates with the colored photograph, while these individual materials are sensitized in such a way that they are light-sensitive in the desired color range, e.g. 400-550 mg, i.e. the entire range 400-550 mg is sensitized by the dye added. As a result, the electrostatic charge decays in these areas. A latent electrostatic image results whose electrostatic charged corresponds to the spectral range from 550-750 mg. By developing these materials with so-called reversal toners, which are repelled by the electrostatic charge present, color separations likewise result, in this case the blue portion, which, as mentioned above, can be inked up with the corresponding colors. The developing itself can in each case be effected in the dry manner by means of toner powder, or in the wet manner by means of particles dispersed or dissolved in a liquid. Toners and developers of this kind are very well known in the electrophotographic field. The number of individual printing plates in the present set of printing plates is not critical, but it must be at least two. Normally, good color reproductions can be achieved with about 3-4 color separations. The present invention, however, also encompasses color separations of 4 or 5, or more, printing plates.

As is known, the quality of the colored image can be increased by preparing a black separation and thus increasing the contours and the so-called stability of the image. A panchromatic coating is naturally necessary for this purpose.

The invention will be further illustrated by reference to the following specific examples:

Example 1 A set of electrophotographic material is prepared which is suitable for 3-color separations and thus for three-color printing:

(1) 2 g. of 2,5-bis(-p-diethylamino-phenyl)-1,3,4-oxadiazole and 2 g. of Lytron 820 are dissolved in 36 g. of methylglycol and mixed with a solution of 4 mg. of brilliant green in 2 ml. of methanol; the solution appears green in transmitted light. It is then whirl-coated onto an aluminum foil. After evaporation of the solvent, the coating is suitable for the electrophotographic production of an image. The light-sensitivity lies substantially between 560 and 700 111,14. In addition, the coating exhibits slight light-sensitivity at 420 m as does the unsensitized foil. This printing foil is suitable for the preparation of a blue separation.

For this purpose, the electrophotographic material is charged, in conventional manner, under a corona device and exposed with the original in contact therewith or episcopically or diascopically, without interposing color filters and then the remaining charged areas of the latent electrostatic image are developed directly in conventional manner with a resin powder. After subsequent fixing, this image is converted into a printing plate. This can be effected by decoating, i.e. dissolving away of the areas which. are not intended to print. At the same time, these areas are rendered hydrophilic, i.e. water-accepting, by treatment e.g. with sodium metasilicate. The toner-bearing image areas are now inked up with blue printing ink.

(II) A solution is prepared, consisting of 1 g. of 2,5- bis(-p-diethylamino-phenyl)-1,3,4-oxadiazole and 1 g. of Lytron in 20 g. of methylglycol. To this solution is added an alcoholic solution of 0.5 mg. of Rhodamine B Extra, C.I. 45170, 2 mg. of Acridine Orange, CI. 46005, and 5 mg. of thioflavine. After whirl-coating this mixture onto an aluminum plate, followed by drying, an electrophotographic coating is obtained whose sensitivity extends from 400 m to 600 ml. After employing the same process steps as stated under (I), this electrophotographic material can be used as a printing plate for the red separation from the original mentioned under (I).

(III) The same procedure is followed as under (II), but using, instead of the sensitizer mixture named therein, 7.5 mg. of thioflavine dissolved in 4 ml. of methanol. The finished coating is light-sensitive in the wavelength range of 400 mg to 500 11111., consequently the image areas of blue characters are discharged, while yellow (and red) print or writing is faithfully reproduced. With this printing plate, very good yellow separations of the original can be prepared by a method analogous to that under (I).

The three-color separations prepared under (I), (II) and (III) above are printed over one another, in conventional manner, in a color printing machine. Very good prints with faithful color reproduction are produced.

The sensitizer Brilliant Green, CI. 42040, as stated under (I), can be replaced by Malachite Green, CI. 42000, or Victoria Pure Blue B, CI. 44045, with equally good results. Similarly, the sensitizer combination in (II) may be replaced by trypaflavine/Acridine Orange, 0.1. 46005. The dye thioflavine stated under (III) may be replaced by the dyes Acridine Yellow G, C.I. 46025, or trypaflavine.

Example 2 The set of material of three electrophotographic plates stated in Example 1 is, for four-color printing, increased by one electrophotographic plate for the black separation. This plate is prepared as follows: 2 g. of 2,5-bis(-pdiethylamino-phenyl)-1,3,4-oxadiazole and 2 g. of Lytron are dissolved in 40 g. of methylglycol. To this is added a solution of 2 mg. of Acridine Yellow G, C.I. 46025, 2 mg. of Acridine Orange, CI. 46005, 1 mg. of Rhodamine B Extra, CI. 45170, and 1 mg. of Brilliant Green, CI. 42040, in 3 ml. of methanol. This solution is applied to a suitable support material, e.g. to an aluminum foil, and the solvent is then evaporated. In this way, a panchromatic coating is obtained. With this, a black separation can be prepared from a colored original according to the procedure of Example 1. This black separation and the three other color separations mentioned in Example 1 are printed over one another. Very good color prints are produced.

Example 3 For a blue-violet color separation inside a set of plate material, the following electrophotographic material may be used:

A solution of 1.5 g. of 2,5-bis(-p-diethylaimino-phenyD- 1,3,4-oxadiazole and 1.5 g. of Lytron 820 in 30 g. of methylglycol is mixed with a solution of 3 mg. of Crystal Violet, C.I. 42555, in 1 ml. of methanol. This mixture is applied to an aluminum foil. After evaporation of the solvent, an electrophotographic coating is obtained whose light sensitivity lies between 500 and 650 m i.e. yellow color is not reproduced.

Crystal Violet, C.I. 42555, can be replaced by the combinations Rhodamine B Extra/ Malachite Green or Rhodamine B Extra/ Brilliant Green with equally good results.

Example 4 As a set of material for five-color printing, the 4 color separations of Example 2 can be used in combination with the following electrophotographic material, which yields a green separation:

2 g. of 2,S-bis(-p-diethylamino-phenyl)-1,3,4-oxadiazole and 2 g. of Lytron are dissolved in 40 g. of methylglycol. To this are added 10 mg. of thioflavine dissolved in 5 ml. of methanol, and 0.67 mg. of Brilliant Green, C.I. 42040, dissolved in 1.34 ml. of methyl alcohol. An aluminum foil is coated with this solution. The light-sensitivity of the electrophotographic coating lies for the most part between 400 and 500 m with a maximum at 450 m and for the lesser part between 590 and 680 mp. with a maximum at 640 I1'l,u. When exposed over an original which contains the colors red, green and blue, green appears, while red and blue disappear. In other words, a green separation is obtained if the method described in Example 1 is followed.

Example 5 A set of material for two-color printing consisting of:

(a) g. of a 50% (by weight) solution of silicone resin in toluene are diluted with g. of toluene. 100 g. of zinc oxide are added to this solution, with stirring. 10 mg. of Fluorescein, (2.1. 45350, and 15 mg. of Acridine Yellow G, C.I. 46025, dissolved in 10 ml. of methanol are then added. The suspension is then ground several times in a colloid mill and then applied to a paper support suitable for electrophotographic purposes. After evaporation of the solvent, a zinc oxide coating is obtained which, according to the method in Example 1, does not reproduce blue characters.

(b) If, instead of the sensitizers fluorescein and acridine yellow, 20 mg. of Basic Pure Blue, CI. 51005, dissolved in 10 ml. of methanol are used, there results, according to the same process, a zinc oxide coating which is sensitive to red. The printing foils thus obtained are hydrophilized in conventional manner. With these, there can be prepared from a two-color original which contains the colors red and blue, a red separation and a blue separation which, when printed over one another, yield a very good image.

Lytron is a copolymer of styrene and maleic acid anhydride produced and marketed by Monsanto Chemicals Ltd., London.

It will be obvious to those skilled in the art that many modifications may be made within the scope of the present invention without departing from the spirit thereof, and the invention includes all such modifications.

What is claimed is:

1. A process for the production of 'multi-colored prints by electrophotographic methods in which a set of at least two electrophotographic materials, each of which comprises a photoconductive layer containing a sensitizer which causes the layer to be light-sensitive only in a selected portion of the spectrum of visible light, are electrostatically charged, separately exposed to the same light image of a colored original to be reproduced, developed, fixed, and converted into printing plates, and the latter are printed over each other on a print-receiving material using printing inks, the colors of which are determined by the selected portions of the spectrum.

2. A process according to claim 1 in which each electrophotographic material is sensitized so that the values of the light-sensitivity for each wavelength of the visible spectrum are substantially inversely proportional to the reflectance values of the printing ink to be used.

3. A process according to claim 1, in which each electrophotographic material is sensitized so that the values of the light-sensitivity for each wavelength of the visible spectral region substantially correspond to the reflectance values of the printing ink to be used, and the material is developed with a reversal developer.

4. A process according to claim 1 in which the sensitizer in each layer is a sensitizing dyestulf.

5. A process according to claim 1 in which each photoconductive layer contains an organic photoconductor.

7 6. A process according to claim 5 in which the photoconductor is an oxadiazole.

7. A process according to claim 1 in which each photocondnctive layer contains an inorganic photoconductor.

8. A process according to claim 7 in which the photoconductor is zinc oxide.

References Cited UNITED STATES PATENTS 2,305,537 12/1942 Lee 101-211 2,962,374 11/ 1960 Dessauer 96l.2 2,962,375 11/ 1960 Schaifert 96- -1.2 3,001,872 9/1961 Kurz 961.8 3,051,569 8/1962 Sugarman et a1. 961.7

Bornarth '961.8 Waldherr 96-32 Behmenburg et al 961.6 T omanek et a1 961.5 Stricklin 961,8 Kaprelian 96l.2 Johnson et a1 96-1.2

NORMAN G. TORCHIN, Primary Examiner 10 I. C. COOPER, III, Assistant Examiner US. Cl. X.R. 

